A surface roughness/shape measuring apparatus or a roundness measuring apparatus (in this specification, such measuring apparatus will be generically referred to as “surface shape measuring apparatus”) measures the surface shape of a measurement object (workpiece) by moving a displacement detector (pickup) equipped with a probe along the surface of the measurement object and by converting the amount of displacement of the probe into an electrical signal which is read into a computer or the like for processing. FIG. 1A shows the basic configuration of such a surface roughness/shape measuring apparatus (refer to patent document 1 listed below).
The surface roughness/shape measuring apparatus 10 measures the surface shape of a workpiece W1 by traversing a stylus 16, i.e., a probe, in the X-axis direction across the surface of the workpiece W1 placed on a work table 17 and thereby detecting, using a displacement detector (pickup) 15, the amount of displacement of the stylus 16 in the Z-axis direction caused by surface irregularities of workpiece W1.
A column 12 is installed vertically on a base 11 placed in a horizontal position. A Z-direction moving part 13 is slidably supported on the column 12, and is moved up and down in the vertical direction by a Z-direction driving means not shown.
An X-arm 14 is supported horizontally on the Z-direction moving part 13. The X-arm 14 is moved forward and backward along the X-axis direction by an X-arm driving means not shown. The displacement detector 15 is attached to the tip of the X-arm 14. With the X-arm 14 moving forward and backward along the X-axis direction, the stylus 16 moves in reciprocating fashion along the X-axis.
The plane where the surface of the base 11 lies is taken as the X-Y plane, and the straight line along which the stylus 16 moves in the X-Y plane is taken as the X-axis. Further, the straight line perpendicular to the X-axis in the X-Y plane is taken as the Y-axis, and the straight line passing through the intersection (origin O) of the Y- and X-axes and oriented perpendicular to the X-Y plane is taken as the Z-axis. The thus defined rectangular space coordinate system is called the O-XYZ coordinate system.
When the stylus 16 is moved along the X-axis while pressing it against the surface of the workpiece W1 by applying a constant force, the stylus 16 is displaced in the Z direction because of the surface irregularities of the workpiece W1. The amount of displacement of the stylus 16 is converted into an electrical signal by a sensor, such as a differential transducer, built into the displacement detector 15.
FIG. 1B is a diagram explaining the amount of displacement detected by the displacement detector 15. The signal value that the displacement detector 15 outputs indicates how far the stylus 16 is elevated as it contacts the workpiece W1. Since the amount of elevation of the stylus 16 differs depending on the relative positional relationship in the Z direction between the workpiece W1 and the displacement detector 15 holding the stylus 16, the amount of displacement, Zi, of the stylus 16 detected by the displacement detector 15 is given as the amount of offset from a reference position (Z0) in the Z direction, which is determined in accordance with the position of the displacement detector 15.
The electrical signal output from the displacement detector 15 is converted by an A/D converter into a digital signal, which is input to a data processing apparatus such as a computer (not shown). In this way, measurement data indicating the surface roughness or surface shape of the workpiece is acquired by the data processing apparatus.
The work table 17 is moved in the Y-axis direction by a work table driving means not shown. By moving the work table 17 in the Y-axis direction, the surface roughness or surface shape of the workpiece W1 can be measured in the X-Y plane by changing the traverse position of the stylus 16 across the surface of the workpiece W1.
FIG. 2A is a diagram showing the basic configuration of a roundness measuring apparatus for measuring the roundness of a cross-sectional shape of a workpiece W2 at least a portion of whose circumference is formed in an arc shape in cross section (see patent document 2 listed below).
When the roundness measuring apparatus 20 measures the arced portion of the cross-sectional circumference of the workpiece W2, the workpiece W2 is placed on a turntable 27 having an axis of rotation oriented parallel to the Z-axis as shown. Then, with the probe 26 pressed against a side face of the workpiece 2, the workpiece W2 is rotated about the center of the arc formed along the cross-sectional circumference of the workpiece W2, thus causing the tip of the probe 26 to trace the circumferential face of the workpiece W2. Since the tip of the probe 26 is displaced as the radius of the arced portion of the cross-sectional circumference of the workpiece W2 changes, the roundness of the arced portion of the cross-sectional circumference of the workpiece W2 can be measured by measuring the change in the amount of displacement.
A column 22 is installed vertically on a base 21 placed in a horizontal position. A Z-direction moving part 23 is slidably supported on the column 22, and is moved up and down in the vertical direction by a Z-direction driving means not shown.
An X-arm 24 is supported horizontally on the Z-direction moving part 23. The X-arm 24 is moved along the X-axis direction by an X-arm driving means not shown. A displacement detector 25 is attached to the tip of the X-arm 24. Then, by moving the Z-direction moving part 23 and hence the X-arm 24, the displacement detector 25 is positioned, and the stylus 26 attached to the displacement detector 25 is brought into contact with the side face of the workpiece W2 placed on the turntable 27.
When the workpiece W2 is rotated while pressing the stylus 26 against the side face of the workpiece W2, the position of the tip of the stylus 26 is displaced in the radial direction of the arc as the radius of the arc formed along the cross-sectional circumference of the workpiece W2 changes. The amount of displacement of the stylus 26 is converted into an electrical signal by a sensor, such as a differential transducer, built into the displacement detector 25.
FIG. 2B is a diagram for explaining the amount of displacement detected by the displacement detector 25. The signal value that the displacement detector 25 outputs indicates how far the stylus 26 pressed against the side face of the workpiece W2 is displaced outwardly in the radial direction of the arc of the side face as the stylus 26 contacts the workpiece W2. Since the amount of outward displacement of the stylus 26 differs depending on the relative positional relationship between the workpiece W2 and the displacement detector 25, the amount of displacement, DRi, of the stylus 26 detected by the displacement detector 25 is given as the amount of offset from a reference radius (DR0), which is determined in accordance with the position of the displacement detector 25.
The electrical signal output from the displacement detector 25 is converted by an A/D converter into a digital signal, which is input to a data processing apparatus such as a computer (not shown). Then, based on the input signal, the data processing apparatus computes the circumferential shape of the cross section of the workpiece W2 to determine the roundness of the arced portion.
[Patent document 1] Japanese Unexamined Patent Publication No. 2002-107144
[Patent document 2] Japanese Unexamined Patent Publication No. H05-231806